Effects of End Mill Helix Angle on Accuracy for Machining Thin-Rib Aerospace Component.
Applied Mechanics and Materials Vol. 315 (2013) pp 773-777
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.315.773
Effects of End Mill Helix Angle on Accuracy for Machining Thin-Rib
Aerospace Component
Raja Izamshah1,a, Yuhazri M. Y.2,b, M. Hadzley1,c M. Amran1,d
and Sivarao Subramonian1,e
1
Department of Manufacturing Process, Faculty of Manufacturing Engineering, Universiti Teknikal
Malaysia Melaka, Malaysia
2
Department of Materials Engineering, Faculty of Manufacturing Engineering, Universiti
Teknikal Malaysia Melaka, Malaysia
a
b
c
[email protected], [email protected], [email protected],
d
[email protected], [email protected]
Keywords: Machining; End Mill; Helix Angle; Surface Error.
Abstract. Accuracy of machined component is one of the challenging tasks for manufacturer. In the
aerospace industry, machining process is widely used for fabrication of unitized-monolithic
component that contains a thin-walled structure. During machining, the cutting forces cause
deflection to the thin-wall section, leading to dimensional form errors that cause the finished part to
be out of specification or failure. Most of the existing research for machining thin-wall component
only concentrated on the process planning and the effects of cutter geometric feature is often
neglected. Tool geometric feature has a direct influence on the cutting performance and should not
be neglected in the machining consideration. This paper reports on the effect of helix angle on the
magnitude of wall deflection. The established effects will be used for the development of high
performance cutting tool for specifically machining thin-wall component.
Introduction
Machining of aerospace structural components involves several thin-wall rib and flange sections.
These thin-wall sections are dictated by design consideration to meet required strength and weight
constraints. The unified parts have remarkable merits including high stiffness and lightness. A
moving body or airfoil of an aircraft consists of a number of slim ribs to satisfy those requirements.
The thickness of these ribs may vary from 100 to 0.1mm.
Kline et al. [1] modelled the milling process of thin-wall rectangular plate element clamped on
three edges taking the effects of a flexible endmill. The interaction between the milling forces and
the structural deformation were neglected on his study. Therefore, their proposed model can only be
applied for the case of relatively rigid workpiece. Sagherian et al. [2] improved Kline’s model by
including the dynamic milling forces and the regeneration mechanism. However, they did not
consider the effect of workpiece deflection on the cutting geometry, i.e. the radial depth of cut.
Later, Tsai and Liao [3] developed an iteration schemes to predict the cutting forces and form error
on thin-wall rectangle plate. The cutting force distribution and the system deflections are solved
iteratively by modified Newton-Raphson method. They made a few assumptions on the size of the
element and their relationship with the transient surface which restricted its applicability. Dynamic
model for milling of low rigidity cantilever plate structure was proposed by Altintas et al. [4].
However, the model did not taken into account the changing of structural properties on the material
removal process.
Budak and Altintas [5] used the beam theory to analyse the form errors when milling using
slender helical endmill for peripheral milling of a cantilever plate structure. The slender helical
endmill is divided into a set of equal element to calculate the form errors acting by the cutting forces
on both tool and the wall. Later in their work [6], they proposed a feed rate scheduling strategy to
reduce the surface errors produced in milling flexible workpiece. However, this approach tends to
sacrifice the productivity as reducing the feed rate will increase the machining time.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 210.48.157.18, Universiti Teknikal Malaysia Melaka, Durian Tunggal, Malaysia-04/03/13,09:53:54)
774
Mechanical & Manufacturing Engineering
From the literatures it shows that, there has been little research done on the evaluation of
machinability in connection with the remarkable change of chatter characteristics based on the
cutter geometrical feature influence such as helix angle. When milling a thin-walled structure,
deflection and chatter vibration of the workpiece due to low stiffness had a negative effect on the
surface integrity and geometric accuracy [7]. Therefore, it is necessary to select optimal cutter
feature when considering those effects. This paper evaluates the machining performance ability
based on cutter helix angle that occurs when side milling of cantilever-shaped materials in order to
find the optimal cutter geometry.
End mill geometrical feature
The geometrical feature of end mill consists of diameter (D), inscribed circle diameter (DW),
number of flute (N), rake angle (γ), clearance angle (η) and helix angle (β) as shown in Fig. 1. Each
of the geometric features has their own specific function. Cutting teeth are located on both the end
face of the cutter and the periphery of the cutter body. The number of flute can affect the end mill
rigidity. Rake angle value will affects the stiffness of the cutting edge and rigidity of the tool. An
end mill with a positive rake angle will improve machinability, thereby producing lower cutting
force and cutting temperature. Increased helix angle induces increased effective rake angle, thereby
improving machinability and accuracy. While a large clearance angle improves surface roughness,
but it is easy to be chipped. On the other side, a small clearance angle is likely to produce noise and
higher surface roughness.
Fig. 1: End mill geometrical features.
In milling process, helical teeth are generally preferred over straight teeth because the tooth is
partially engaged with workpiece as it rotates. Helix angle is the angle formed by a line tangent to
the helix and a plane through the axis of the cutter or the cutting edge angle which a helical cutting
edge makes with a plane containing the axis of a cylindrical cutter. The helix helps move chips up
and out of the cutting zone and also generates an excellent surface finish. Helix angle permits
several teeth to cut simultaneously which result in smoother cutting action. Standard end mill helix
angle for common machining process are in the ranges between 30° to 45° for sharpness and cutting
edge strength. This is adequate for carbon steels, some tool steels or even light finishing passes in
aluminum. However, when machining stainless steels, a sharper cutting edge needs to be employed
as to lessen the work hardening effects and promote a more free cutting action. This is where a 45°
helix angle does well because there will still be some cutting edge strength along with appropriate
sharpness needed. This angle is also good for aluminum engaging in a deeper slot or periphery cut.
However chip space available decrease as core diameter of cutter increase. Chip disposal also
depends somewhat on the helix angle, being easier for high helix angle. However, as helix angle
increases, cutting edge becomes weaker. A sharp edge and high rake angles are needed to separate a
chip from the parent material. Normal rake of 10° to 14° degree with combine 30° of helix angle
provides the best purpose effective rake suitable for most material.
Applied Mechanics and Materials Vol. 315
775
Experimental design and setup
Using a CNC tool cutter grinder Michael Deckel S20 Turbo, design parameter and proper wheel
geometries were determined to produce the end mill. A two flutes carbide flat end mill with a
diameter of 10 mm and total length of 70 mm were fabricated to analyze the effects of variable helix
angle on part deflection as depicted in Table 1.
Table1: End mill specification
Material
Carbide
Diameter
10 mm
Total length
70 mm
Flute length
40 mm
Bottom
Flat
No. of flute
2
In this experiment, the end mill were design to maintain varying helix angles of 25o, 30o, 35o, 40o
and 45o with a constant rake angle of 8o and clearance angle of 6o, respectively as shown in Fig. 2.
Others design factors were held constant. Each of the fabricated end mills were then measured using
a tool microscope for accuracy.
Fig. 2: Various end mill helix angles.
Fig. 3: Measuring points for the workpiece.
Table 2: Workpiece and machining parameters use in the experiment
Workpiece
Type
Aluminum alloy 7076
Wall thickness
4 mm
Length
100 mm
Height with base
50 mm
Machining parameter
Cutting speed
2400 rpm
Axial depth of cut
38 mm
Radial depth of cut
2 mm
Feed
0.1
mm/tooth
The workpiece use in the experiment was Aluminum alloy 7076 series and was pre-machined to
4 mm thickness for each sample using a step machining approach to minimize the surface error. The
workpiece dimension and the measured location for the surface dimensional error are shown in Fig.
3. Side milling cutting tests was conducted using a 5-axis Deckel Maho milling machine to
investigate the effects of wall deflection with respect to the changes in helix angle. After the
machining process, the magnitudes of wall deflection were measured using Carl Zeiss coordinate
measuring machine. There were total of 114 measurement points of each sample. Six parallel lines
positioned at 5mm, 10mm, 15mm, 20mm, 25mm and 30mm respectively from the top surface. Each
776
Mechanical & Manufacturing Engineering
line consists of 19 points with 5mm distance. The wall is to be reduces to 2 mm in thickness with an
axial depth of cut of 38 mm. Table 2 shows the workpiece and cutting parameter for the side milling
experiment.
(a)
(b)
(c)
(d)
(e)
Fig. 4: 3D graph of part deflection for variable helix angles for (a) 25o, (b) 30o, (c) 35o, (d) 40o and
(e) 45o.
Experimental Results
Fig. 4 shows the magnitude of wall deflection due to varying helix angles for 25o, 30o, 35o, 40o
and 45o with a constant rake angle of 8o and clearance angle of 6o, respectively. From the graphs, it
clearly shows a variation value in wall deflection due to varying of helix angle. The form errors for
varying of helix angle are smallest at the bottom compared at the top of the part where the wall
Applied Mechanics and Materials Vol. 315
777
flexibility decreases. Due to the decreasing stiffness of the wall as a result of material removal, there
is an increasing value of form errors between two regions (start and end) in the feed direction. To a
large extent, the more flexible the wall, the higher surface errors result during cutting.
From the graphs, it shows that 40º and 45o helix angle gives the least surface error, followed by
35º and 25º. It shows that end mills with small helix angles tend to cause high cutting forces which
deflect the workpiece due to chatter and reduction of the period when the end mill flute engage with
the workpiece material.
Conclusion
From the experiment it shows that end mill helix angle affects the magnitude of wall deflection
during machining thin-wall components. The magnitude of wall deflection decreases as the value of
helix angle increase. End mills with helix angle ranging from 40o to 45o were very effective and
would be recommended for the case of milling thin-wall application. The findings from this
experiment can be use for the development of high performance cutting tool for specifically
machining thin-wall components.
Acknowledgement
The authors gratefully acknowledge Malaysia Ministry of Higher Education for supported the
work under the Fundamental Research Grant Scheme No. FRGS/2012/FKP/TK01/03/2 F00134.
References
[1]
W.A Kline, R.E. DeVor and I.A. Shareef, The prediction of surface accuracy in end milling,
ASME J. Eng. Ind. 104 (1982) 272-278.
[2]
R. Sagherian and M.A. Elbestawi, A simulation system for improving machining accuracy in
milling, Computers in Industry 14 (1990) 293-305.
[3]
J.S Tsai and C.L. Liao, Finite element modeling of static surface errors in the peripheral
milling of thin-walled wall, J. of Mat. Processing Tech. 94 (1999) 235-246.
[4]
Y. Altintas, D. Montgomery and E. Budak, Dynamic peripheral milling of flexible structures,
J. of Eng. for Industry (Transaction of the ASME) 114 (1992) 137-145.
[5]
E. Budak and Y. Altintas, Peripheral milling conditions for improved dimensional accuracy,
Int. J. of Mach. Tools and Manufacture 34 (1994) 907-918.
[6]
E. Budak and Y. Altintas, Modeling and avoidance of static form errors in peripheral milling
of plates, Int. J. of Mach. Tools and Manufacture 35 (3) (1995) 459–476.
[7]
H. Ning, W. Zhigang, J. Chengyu and Z. Bing, Finite element method analysis and control
stratagem for machining deformation of thin-walled components, J. of Mat. Processing Tech.,
Vol. 139 (2003) 332-336.
© (2013) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMM.315.773
Effects of End Mill Helix Angle on Accuracy for Machining Thin-Rib
Aerospace Component
Raja Izamshah1,a, Yuhazri M. Y.2,b, M. Hadzley1,c M. Amran1,d
and Sivarao Subramonian1,e
1
Department of Manufacturing Process, Faculty of Manufacturing Engineering, Universiti Teknikal
Malaysia Melaka, Malaysia
2
Department of Materials Engineering, Faculty of Manufacturing Engineering, Universiti
Teknikal Malaysia Melaka, Malaysia
a
b
c
[email protected], [email protected], [email protected],
d
[email protected], [email protected]
Keywords: Machining; End Mill; Helix Angle; Surface Error.
Abstract. Accuracy of machined component is one of the challenging tasks for manufacturer. In the
aerospace industry, machining process is widely used for fabrication of unitized-monolithic
component that contains a thin-walled structure. During machining, the cutting forces cause
deflection to the thin-wall section, leading to dimensional form errors that cause the finished part to
be out of specification or failure. Most of the existing research for machining thin-wall component
only concentrated on the process planning and the effects of cutter geometric feature is often
neglected. Tool geometric feature has a direct influence on the cutting performance and should not
be neglected in the machining consideration. This paper reports on the effect of helix angle on the
magnitude of wall deflection. The established effects will be used for the development of high
performance cutting tool for specifically machining thin-wall component.
Introduction
Machining of aerospace structural components involves several thin-wall rib and flange sections.
These thin-wall sections are dictated by design consideration to meet required strength and weight
constraints. The unified parts have remarkable merits including high stiffness and lightness. A
moving body or airfoil of an aircraft consists of a number of slim ribs to satisfy those requirements.
The thickness of these ribs may vary from 100 to 0.1mm.
Kline et al. [1] modelled the milling process of thin-wall rectangular plate element clamped on
three edges taking the effects of a flexible endmill. The interaction between the milling forces and
the structural deformation were neglected on his study. Therefore, their proposed model can only be
applied for the case of relatively rigid workpiece. Sagherian et al. [2] improved Kline’s model by
including the dynamic milling forces and the regeneration mechanism. However, they did not
consider the effect of workpiece deflection on the cutting geometry, i.e. the radial depth of cut.
Later, Tsai and Liao [3] developed an iteration schemes to predict the cutting forces and form error
on thin-wall rectangle plate. The cutting force distribution and the system deflections are solved
iteratively by modified Newton-Raphson method. They made a few assumptions on the size of the
element and their relationship with the transient surface which restricted its applicability. Dynamic
model for milling of low rigidity cantilever plate structure was proposed by Altintas et al. [4].
However, the model did not taken into account the changing of structural properties on the material
removal process.
Budak and Altintas [5] used the beam theory to analyse the form errors when milling using
slender helical endmill for peripheral milling of a cantilever plate structure. The slender helical
endmill is divided into a set of equal element to calculate the form errors acting by the cutting forces
on both tool and the wall. Later in their work [6], they proposed a feed rate scheduling strategy to
reduce the surface errors produced in milling flexible workpiece. However, this approach tends to
sacrifice the productivity as reducing the feed rate will increase the machining time.
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 210.48.157.18, Universiti Teknikal Malaysia Melaka, Durian Tunggal, Malaysia-04/03/13,09:53:54)
774
Mechanical & Manufacturing Engineering
From the literatures it shows that, there has been little research done on the evaluation of
machinability in connection with the remarkable change of chatter characteristics based on the
cutter geometrical feature influence such as helix angle. When milling a thin-walled structure,
deflection and chatter vibration of the workpiece due to low stiffness had a negative effect on the
surface integrity and geometric accuracy [7]. Therefore, it is necessary to select optimal cutter
feature when considering those effects. This paper evaluates the machining performance ability
based on cutter helix angle that occurs when side milling of cantilever-shaped materials in order to
find the optimal cutter geometry.
End mill geometrical feature
The geometrical feature of end mill consists of diameter (D), inscribed circle diameter (DW),
number of flute (N), rake angle (γ), clearance angle (η) and helix angle (β) as shown in Fig. 1. Each
of the geometric features has their own specific function. Cutting teeth are located on both the end
face of the cutter and the periphery of the cutter body. The number of flute can affect the end mill
rigidity. Rake angle value will affects the stiffness of the cutting edge and rigidity of the tool. An
end mill with a positive rake angle will improve machinability, thereby producing lower cutting
force and cutting temperature. Increased helix angle induces increased effective rake angle, thereby
improving machinability and accuracy. While a large clearance angle improves surface roughness,
but it is easy to be chipped. On the other side, a small clearance angle is likely to produce noise and
higher surface roughness.
Fig. 1: End mill geometrical features.
In milling process, helical teeth are generally preferred over straight teeth because the tooth is
partially engaged with workpiece as it rotates. Helix angle is the angle formed by a line tangent to
the helix and a plane through the axis of the cutter or the cutting edge angle which a helical cutting
edge makes with a plane containing the axis of a cylindrical cutter. The helix helps move chips up
and out of the cutting zone and also generates an excellent surface finish. Helix angle permits
several teeth to cut simultaneously which result in smoother cutting action. Standard end mill helix
angle for common machining process are in the ranges between 30° to 45° for sharpness and cutting
edge strength. This is adequate for carbon steels, some tool steels or even light finishing passes in
aluminum. However, when machining stainless steels, a sharper cutting edge needs to be employed
as to lessen the work hardening effects and promote a more free cutting action. This is where a 45°
helix angle does well because there will still be some cutting edge strength along with appropriate
sharpness needed. This angle is also good for aluminum engaging in a deeper slot or periphery cut.
However chip space available decrease as core diameter of cutter increase. Chip disposal also
depends somewhat on the helix angle, being easier for high helix angle. However, as helix angle
increases, cutting edge becomes weaker. A sharp edge and high rake angles are needed to separate a
chip from the parent material. Normal rake of 10° to 14° degree with combine 30° of helix angle
provides the best purpose effective rake suitable for most material.
Applied Mechanics and Materials Vol. 315
775
Experimental design and setup
Using a CNC tool cutter grinder Michael Deckel S20 Turbo, design parameter and proper wheel
geometries were determined to produce the end mill. A two flutes carbide flat end mill with a
diameter of 10 mm and total length of 70 mm were fabricated to analyze the effects of variable helix
angle on part deflection as depicted in Table 1.
Table1: End mill specification
Material
Carbide
Diameter
10 mm
Total length
70 mm
Flute length
40 mm
Bottom
Flat
No. of flute
2
In this experiment, the end mill were design to maintain varying helix angles of 25o, 30o, 35o, 40o
and 45o with a constant rake angle of 8o and clearance angle of 6o, respectively as shown in Fig. 2.
Others design factors were held constant. Each of the fabricated end mills were then measured using
a tool microscope for accuracy.
Fig. 2: Various end mill helix angles.
Fig. 3: Measuring points for the workpiece.
Table 2: Workpiece and machining parameters use in the experiment
Workpiece
Type
Aluminum alloy 7076
Wall thickness
4 mm
Length
100 mm
Height with base
50 mm
Machining parameter
Cutting speed
2400 rpm
Axial depth of cut
38 mm
Radial depth of cut
2 mm
Feed
0.1
mm/tooth
The workpiece use in the experiment was Aluminum alloy 7076 series and was pre-machined to
4 mm thickness for each sample using a step machining approach to minimize the surface error. The
workpiece dimension and the measured location for the surface dimensional error are shown in Fig.
3. Side milling cutting tests was conducted using a 5-axis Deckel Maho milling machine to
investigate the effects of wall deflection with respect to the changes in helix angle. After the
machining process, the magnitudes of wall deflection were measured using Carl Zeiss coordinate
measuring machine. There were total of 114 measurement points of each sample. Six parallel lines
positioned at 5mm, 10mm, 15mm, 20mm, 25mm and 30mm respectively from the top surface. Each
776
Mechanical & Manufacturing Engineering
line consists of 19 points with 5mm distance. The wall is to be reduces to 2 mm in thickness with an
axial depth of cut of 38 mm. Table 2 shows the workpiece and cutting parameter for the side milling
experiment.
(a)
(b)
(c)
(d)
(e)
Fig. 4: 3D graph of part deflection for variable helix angles for (a) 25o, (b) 30o, (c) 35o, (d) 40o and
(e) 45o.
Experimental Results
Fig. 4 shows the magnitude of wall deflection due to varying helix angles for 25o, 30o, 35o, 40o
and 45o with a constant rake angle of 8o and clearance angle of 6o, respectively. From the graphs, it
clearly shows a variation value in wall deflection due to varying of helix angle. The form errors for
varying of helix angle are smallest at the bottom compared at the top of the part where the wall
Applied Mechanics and Materials Vol. 315
777
flexibility decreases. Due to the decreasing stiffness of the wall as a result of material removal, there
is an increasing value of form errors between two regions (start and end) in the feed direction. To a
large extent, the more flexible the wall, the higher surface errors result during cutting.
From the graphs, it shows that 40º and 45o helix angle gives the least surface error, followed by
35º and 25º. It shows that end mills with small helix angles tend to cause high cutting forces which
deflect the workpiece due to chatter and reduction of the period when the end mill flute engage with
the workpiece material.
Conclusion
From the experiment it shows that end mill helix angle affects the magnitude of wall deflection
during machining thin-wall components. The magnitude of wall deflection decreases as the value of
helix angle increase. End mills with helix angle ranging from 40o to 45o were very effective and
would be recommended for the case of milling thin-wall application. The findings from this
experiment can be use for the development of high performance cutting tool for specifically
machining thin-wall components.
Acknowledgement
The authors gratefully acknowledge Malaysia Ministry of Higher Education for supported the
work under the Fundamental Research Grant Scheme No. FRGS/2012/FKP/TK01/03/2 F00134.
References
[1]
W.A Kline, R.E. DeVor and I.A. Shareef, The prediction of surface accuracy in end milling,
ASME J. Eng. Ind. 104 (1982) 272-278.
[2]
R. Sagherian and M.A. Elbestawi, A simulation system for improving machining accuracy in
milling, Computers in Industry 14 (1990) 293-305.
[3]
J.S Tsai and C.L. Liao, Finite element modeling of static surface errors in the peripheral
milling of thin-walled wall, J. of Mat. Processing Tech. 94 (1999) 235-246.
[4]
Y. Altintas, D. Montgomery and E. Budak, Dynamic peripheral milling of flexible structures,
J. of Eng. for Industry (Transaction of the ASME) 114 (1992) 137-145.
[5]
E. Budak and Y. Altintas, Peripheral milling conditions for improved dimensional accuracy,
Int. J. of Mach. Tools and Manufacture 34 (1994) 907-918.
[6]
E. Budak and Y. Altintas, Modeling and avoidance of static form errors in peripheral milling
of plates, Int. J. of Mach. Tools and Manufacture 35 (3) (1995) 459–476.
[7]
H. Ning, W. Zhigang, J. Chengyu and Z. Bing, Finite element method analysis and control
stratagem for machining deformation of thin-walled components, J. of Mat. Processing Tech.,
Vol. 139 (2003) 332-336.